Micromechanical acceleration sensors that are manufactured on a mass-produced basis, for example, for applications in the automotive sector or the consumer sector, are known in the existing art. In the automotive sector in particular, one of the great challenges is to design such sensors with maximum vibration robustness. For this reason, it is desirable with regard to a micromechanical acceleration sensor to implement a transfer function in which low frequencies in the region of the useful band (typical bandwidth approx. 10 to approx. 400 Hz) are transferred “smoothly,” and higher frequencies due to undesired vibratory excitations are effectively suppressed.
For suppression of high deflection amplitudes, acceleration sensors are in general hermetically encapsulated in a sensor cavity having a relatively high internal pressure of several 100 mbar; higher-viscosity gases, for example neon, are often used for this. The enclosed gas ensures a high degree of damping of the micromechanical structure. The aforesaid sensors are usually over-damped, so that a resonance exaggeration also does not occur in the range of the mechanical natural frequency f0 (typically several kilohertz).
The damping thus produces a mechanical low-pass behavior, but the amplitude suppression of at most 20 dB per frequency decade is relatively weak. A steeper decrease in the transfer function, of 40 dB per frequency decade, is achieved above the natural frequency. In order to increase vibration resistance it is therefore desirable to design a sensor having the lowest possible natural frequency, for example 1 kHz or even lower. In the case of a lateral acceleration sensor, however (i.e. one having sensing sensitivity parallel to a chip plane), according to the existing art there are tight limits on this effort.
These sensors can be described in simplified fashion as a spring-mass system whose deflection can be calculated in accordance with Hooke's law as:m*a=k*x  (1)    m=mass    a=acceleration    k=spring constant    x=deflection.
The deflection x is obtained as:x=m*a/k  (2)or, reformulated using the natural frequency f0:x=a/(4*π2*f02)  (3)
The deflection amplitude x thus increases as the square of the decrease in resonant frequency.
A typical sensor for so-called “low-g” applications, with measurement ranges of a few g (e.g. for ESP or hill start assist), has a natural frequency of 3 kHz and deflects approximately 28 nm upon an acceleration of 1 g. With a sensor having a natural frequency of 1 kHz this value would already increase to approx. 250 nm. Because acceleration sensors even for low-g applications must have a dynamic range or overload robustness of 20 g or even 50 g (i.e. must not mechanically hit or “clip”), an acceleration sensor having a 50 g clipping acceleration would need to permit a deflection of more than 12 μm.
Typical gaps in the plate capacitor assemblages usually used between the movable and fixed electrodes of capacitive acceleration sensors are, however, only approx. 2 μm in size. Widening the gap to 12 μm would cause the sensitivity of the individual electrodes to decrease by a factor of approximately 36, since the sensitivity dC/dx has an inverse square dependence on the gap.
It would also be possible to implement considerably fewer electrodes on a given area, so that the total sensitivity would in fact decline even more sharply and the signal to noise ratio would thus be intolerably poor.
An alternative to plate electrodes is represented by so-called comb electrodes, which are known e.g. from DE 10 2006 059928 A1 and in which larger deflections are possible. The damping forces in comb electrodes are substantially smaller, however, so that it might not be possible to implement an overcritically damped sensor. In addition, the requisite large deflections present difficult challenges in terms of spring design, since the springs not only become soft in a drive direction but also undesirably lose considerable stiffness, and thus overload resistance, in the transverse directions. The constrained correlation between natural frequency and mechanical sensitivity therefore sets very stringent limits on options for lowering the resonant frequency of micromechanical lateral acceleration sensors.
Patent document EP 0 244 581 B1 and EP 0 773 443 B1 furthermore discusses Z-acceleration sensors using the seesaw principle. An advantage with Z seesaws is the fact that there is no constrained correlation between natural frequency and mechanical sensitivity. This is because the natural frequency f0 of the sensor depends on the torsional stiffness kt of the springs and on the moment of inertia J of the seesaw around the rotation axis, according to the following equation:f02=[1/(2*π)2]*kt/J  (4).
The mechanical sensitivity, i.e. the rotation angle α for a given acceleration a, is also defined by the torsional stiffness kt of the springs, but additionally by the mass asymmetry δm and by the torque resulting therefrom:δα/δa≈δm*rm/kt  (5)    δα/δa=mechanical sensitivity    δm=mass asymmetry    rm=distance between center of mass of the asymmetrical mass and the torsion axis    kt=torsional stiffness of the springs.
The natural frequency f0 can be reduced by decreasing the torsional spring stiffness kt or by increasing the total moment of inertia J of the seesaw. If the mass asymmetry δm of the sensor is also reduced, the mechanical sensitivity of the sensor can nevertheless be kept almost arbitrarily low, and therefore need not create any limitations in terms of wide latitude when designing the sensor. This principle of using a Z seesaw as a mechanical low-pass is described in DE 10 2006 032 194 A1.
Also believed to be understood are Z seesaws that function simultaneously as X and Y lateral acceleration sensors, for example as from DE 10 2008 001 442 A1 in which a so-called “single-mass oscillator” is disclosed. An advantage of these sensors is their very compact design: with only a single seismic mass and only two springs it is possible to implement a three-channel acceleration sensor that not only executes the normal rotational motion around the Y axis in the context of a Z acceleration, but also can deflect linearly in an X direction upon an X acceleration and reacts to a Y acceleration with a rotation around the Z axis.
A mass asymmetry with respect to the rotation axis is utilized for Y sensing; and, analogously to the operating principle of the Z seesaw, the mechanical sensitivity and natural frequency around the Z axis are not rigidly correlated. But the design of the conventional single-mass oscillator is not optimal if the intention is to implement only a single-channel lateral sensor, i.e. a pure Y sensor, since the structure is deflectable in all spatial directions and can therefore deliver undesired signals (crosstalk) in the context of interference accelerations in the two directions perpendicular to the useful direction, in particular with a very large overload when the structure mechanically clips or hits.